Variable inductors are commonly needed in high power, high frequency applications. In the past, variable inductors have been provided in high power, high frequency applications by both mechanically and electrically or magnetically controlled systems.
One typical mechanically controlled variable inductor system has been to use an array of fixed air core solenoids that are switched in and out of a circuit as needed. In such cases, the element with the smallest inductance is typically connected to the circuit by a mechanical contact that can slide along the solenoid; so that, continuous values of inductance can be obtained.
Typical electrically or magnetically controlled variable inductor systems can be constructed with external magnets. One example which is used in a device called a paraformer is described in I. M. Gottlieb, Regulated Power Supplies 4ed., published by Tab Books, Blue Ridge Summit, Pa., 1992, on p.199-205. In this example, one horseshoe-shaped electromagnet (primary) is placed along the axis of a second electromagnet (secondary). However, the poles are rotated by a quarter-circle from each other. In this configuration, an adjustable electrical current through the primary, controls the permeability of the second magnet. This arrangement reduces, but does not eliminate the pickup between the secondary and primary coils. However, because of the size and number of turns required in the secondary, such a configuration is frequently unsuitable for high frequency operation. Another example is given in U.S. Pat. No. 2,882,392, issued to W. F. Sands on Apr. 14, 1959, in which varying the inductance value of a ferrite core inductor is accomplished by mechanically moving the core in and out of a coil through which an electrical current is passed. However, mutual inductance or pick up between the ferrite inductor and the external coil causes a significant loss of power through the inductor, particularly at high frequencies. The tuning speed is also limited by the mechanical movement of the coil or the core.
While these mechanical and electrical or magnetic systems have been used in the past, they do have several serious drawbacks. A major drawback with the mechanical system is that the switching time can be quite long. In some circumstances it can be on the order of several seconds, depending on the precision of the set inductance. Secondly, the mechanical contacts can wear and become unreliable after extended use or use in adverse environments. Thirdly, the large number of inductors necessary to cover the required inductance range often make the system relatively large and expensive.
With the electrically or magnetically controlled variable inductors, the control circuits are often adversely affected by having the high frequency signal induced on the control circuit. This often arises when the magnetic field, caused by the RF signal, and the controlled magnetic field are parallel. Secondly, the electric or magnetic systems are often subject to high signal loss, unless it is minimized by a typically complex and expensive circuitry. Such additional circuitry can contribute to increased size and expense, loss of high frequency power, and causes heating of the adjacent electrical components.
Consequently, there exists a need for improvement in variable inductor systems for high power, high frequency applications which do not have the long switching time, the unreliability and signal loss, or complex and expensive control circuitry which are associated with typical prior art systems.